1. Field of the Invention
The present invention relates to transparent semiconductor devices. Specifically, the present invention relates to field-effect thin-film transistors and thin-film diodes having amorphous-oxide semiconductor-layers.
2. Description of the Related Art
Field-effect transistors (FETs) are three-terminal devices each having a gate electrode, a source electrode, and a drain electrode. An FET is an electronically active device for switching a current between the source electrode and the drain electrode by applying a voltage to the gate electrode to control the current flowing in a channel layer. In particular, an FET having a channel layer of a thin film disposed on an insulating substrate of ceramics, glass, or plastic is called a thin-film transistor (TFT).
TFTs are fabricated by using thin-film technology and advantageously can be readily formed on a substrate having a relatively large area. Therefore, TFTs are widely used as driving devices for flat-panel displays such as liquid-crystal displays. In other words, the ON/OFF of each image pixel of an active liquid-crystal display (ALCD) is controlled by a TFT fabricated on a glass substrate. Further, TFTs will be effective for driving pixel currents of future high-performance organic light-emitting diode (OLED) displays. In addition, a TFT circuit capable of driving/controlling an entire image has been fabricated on a substrate at the periphery of an image display region to allow a liquid-crystal display to achieve higher performance.
At present, metal-insulator-semiconductor field-effect transistors (MIS-FETs) are broadly used as TFTs. In MIS-FET devices, polycrystalline silicon films or amorphous silicon films are used as a channel layer. Practically, amorphous-silicon TFTs are used for driving pixels, and high-performance polycrystalline-silicon TFTs are used for driving/controlling entire images.
However, existing TFTs, such as amorphous-silicon TFTs and polycrystalline-silicon TFTs, are opaque to visible-range light.
Recently, use of an oxide for a channel layer of a transistor transparent to visible-range light has been considered.
For example, TFTs having a channel layer formed of a conductive and transparent thin film of polycrystalline oxide containing ZnO as the main component have been actively developed. The above-mentioned thin film can be formed at a relatively low temperature on a substrate such as a plastic sheet or film. However, materials containing ZnO as the main component cannot form a stable amorphous phase at room temperature. Consequently the materials form a polycrystalline phase. Therefore, the electron mobility of the resulting thin film cannot be increased due to the scattering at the interfaces of the polycrystalline particles. Additionally, the shapes of the polycrystalline particles and interconnection differ largely, depending on the methods of forming the film, resulting in unevenness in the characteristics of the TFT. Furthermore, the transparency may be decreased by the scattering of light at the interfaces of the polycrystalline particles.
Recently, a thin-film transistor using an amorphous oxide of an InGaZnO system has been reported (K. Nomura, et al., Nature, Vol. 432, 25 Nov. 2004, pp. 488-492).
FIG. 9 is a diagram showing the structure of this thin-film transistor.
A channel layer 31 of a-InGaZnO4 is disposed on a glass substrate 30, and a gate-insulating layer 32 of Y2O3 is disposed on the channel layer 31. Reference numerals 33, 34, and 35 denote a source electrode, a drain electrode, and a gate electrode of indium-tin-oxide (ITO), respectively.
This transistor can be fabricated on a plastic or glass substrate at room temperature. The transistor has a field-effect mobility of about 6 to 9 and normally-OFF transistor characteristics. In addition, the transistor is transparent to visible light.
The thin-film transistor using the channel layer of the above-mentioned oxide has a transparency of about 80% to visible light. However, higher transparency is often desired, depending on the intended use. For example, when the transistor is disposed on a display surface, backplane, or glass of an automobile, higher transparency is preferred.
Furthermore, some thin-film transistors may appear to have color due to reflected light, depending on the constitution of the devices (e.g., thickness of each layer). In addition, designs of electronic devices may be restricted for retaining the transparency.